In the field of remotely interrogatable passive transponders, recourse may currently be had to delay lines produced on lithium niobate and operating in their wireless version in the ISM band centered at 2.45 GHz (imposed by the band requirement necessary for the associated spectral function) on the basis of electrodes generating acoustic waves.
This type of transponder can notably be used for detecting variations in physical parameters such as pressure, temperature, etc.
This poses the difficulty of producing delicate technological devices through the dimensions of the electrode combs of the transducers (0.4 μm typically, i.e. a wavelength of 1.6 μm for propagation speeds of 3500 to 3800 m.s−1). These devices must be produced on lithium niobate to obtain electromechanical couplings that are compatible with the corresponding mode of interrogation (Ks2 at least equal to 5%). These structures therefore exhibit large drifts in temperature which may pose a problem in certain cases.
Moreover, to produce the suitable delays (that is to say compatible with the permitted bandwidth), these delay lines exhibit dimensions of the order of 6 to 10 mm, excluding encapsulation, possibly turning out to be critical for applications demanding sensors of small dimension.
Thus for an application of this principle at lower frequencies, the analysis possibly being considered purely linear as a first approximation, the corresponding dimensions are dependent on the frequency scale ratio. And for a 434-MHz reflective delay line application, the distance required to generate the sought-after delays is of the order of 6 times that at 2.45 GHz.
Moreover, the band permitted for emission at 434 MHz is relatively eight times as narrow, thus imposing still larger propagation distances.
2.45-GHz devices used as transponders and comprising a wideband transducer emitting Rayleigh surface waves have also already been proposed. The latter are reflected on reflecting arrays and are detected by the emitter transducer, thus returning a signal of barcode type (the presence of a reflecting group corresponding to logic 1, its absence to 0). This response may be modulated in intensity by connecting a load of variable electrical properties.
Another approach can also consist in using two independent resonators, one of which sees its properties modulated by a load connected in parallel or in series. The latter approach is, however, tricky to implement effectively insofar as whatever the electrical association adopted, the load inevitably influences both the resonators forming the sensor/transponder (it is probably not possible to envisage a measurement with a lone resonator disturbed by the load, the measurement always requiring a reference intended to eliminate the correlated disturbance sources).